Forty years ago, on July 20, 1969, a low decade reached a glorious apogee when Neil Armstrong steered Eagle over a lunar rock field and, with 17 seconds of fuel left, gently set his spidery spacecraft down on the Sea of Tranquility--the first time human beings had landed on another celestial body. Strangely, Apollo 11 never quite seized the public imagination the way Apollo 8 did with its Christmas Eve circumnavigation of the Moon and those stunning “Earth-rise” photos. Three Apollo missions later (and nine months before the high drama of Apollo 13), Americans were accustomed to success in spaceflight. There was nothing automatic about that first lunar landing, though, which was a hair-raiser until touchdown. On its 40th anniversary, it’s worth remembering what an extraordinary accomplishment Apollo 11 was.

Perhaps the most impressive thing about the entire Apollo program was the creativity that went into it. No one knew how to do this in 1961, when President Kennedy announced that the United States would land a man on the Moon and return him safely to Earth before the decade was out. The booster rocketry wasn’t yet designed. The path to the Moon--direct ascent, earth orbit rendezvous, lunar orbit rendezvous?--was undetermined. The stack of vehicles to do the job hadn’t been imagined. The Apollo command module, Columbia, was the most complex piece of machinery ever built; millions of parts had to be invented from scratch. The same was true for the lunar module, Eagle, the first craft ever built to fly solely in space: Should it have three legs or four? Where would the windows go? Would the astronauts sit or stand? How do you keep the pilots from punching a hole in the Eagle’s .0000833 inch-thick nickel-steel skin--a hull the thickness of three sheets of aluminum foil?

Everything about the Apollo program--hardware, software, navigation techniques, mission rules and procedures--had to be invented: a stunning exercise in intellectual creativity and engineering prowess. And it was all done without today’s sophisticated computers--the computer on-board Eagle had less computing power than your standard phone today.

In ‘‘The Right Stuff,’’ Tom Wolfe justifiably celebrated the courage and skill of the astronaut corps, spiritual qualities that ought to remain an inspiration today. Yet in the retrospect of 40 years, what seems equally impressive is the sheer volume of creative ideas that made possible Neil Armstrong’s epic transmission--“Houston, Tranquility Base here. The Eagle has landed”--and Buzz Aldrin’s word-portrait of the Moon’s “magnificent desolation.” As we ponder the accomplishment of Apollo 11, the thought that bears repeating is, “No one knew how to do any of this, eight years before it happened.”

On this anniversary, it’s also worth reflecting on why we stopped pushing out into space and what that’s meant. At the height of the space race, it was simply assumed that, after conquering the Moon (and perhaps building a permanent base there), there would be a Mars mission, which was thought doable by the end of the 20th century, if not earlier. Yet Congress decreed that we stop exploring the Moon with Apollo 17; we can’t get back with the equipment we have now; Mars remains an unfocused dream; and the next men on the Moon (or beyond) could be Chinese.

The lowness of another low decade, the 1970s, had something to do with America’s failure to keep pushing the outside of the envelope in space, I suspect. As in the spiritual life, so in public life: if we look down, or look around, but don’t look up, the human spirit withers a bit. After a season of withering, we find it difficult to imagine ourselves as creatures called to transcend ourselves. So we turn inward, become self-absorbed, and end up, like contemporary Europe--trapped in a crisis of civilizational morale, unable to summon the moral energy to create future generations.

God made us for adventure and discovery. Abandoning the great adventure of manned space exploration was a serious mistake, for America and for the human future.

George Weigel is Distinguished Senior Fellow of the Ethics and Public Policy Center in Washington, D.C.